Solution growth of epitaxial layers of semiconductor material



Aug. 26, 1969 1. MELNGAIL'IS ETAI- 3,463,630

SOLUTION GfiOWTH OF EPITAXIAL LAYERS 0F SEMICONDUCTOR MATERIAL Filed Nov. 25, 1966 CONTACT MELT 30o DECANTED INVENTORS IVARS MELNGAIUS ARTHUR R. CALAWA L BY. F166 5 1b 15 F0 TIME (MIN) NEY United States Patent Int. Cl. H011 7/46 U.s.c1.14s--172 16 Claims This invention relates to methods and means for growing a thin semiconductor layer on a substrate, and more particularly, to epitaxial growth of a semiconductor layer from the liquid state.

Heretofore, epitaxial growth has been accomplished in a vapor phase in which the epitaxial layer is formed by vapor deposition and from the liquid phase in a gaseous environment. Vapor phase epitaxial growth is well known and employed extensively throughout the industry. It requires careful control of the ingredients in the vapor, as well as the vapor pressure and temperature. Accordingly, heretofore both vapor phases and liquid phase epitaxial growth have been accomplished in a gaseous atmosphere, which is precisely controlled as to a composition, temperature and pressure. For example, an epitaxial film of GaAs is formed on a substrate of p-type GaAs by dissolving GaAs in tin and then letting the molten tin run over the exposed surfaces of the GaAs substrate, all while in a gaseous atmosphere. As the melt cools, the GaAs precipitates from the melt and epitaxial growth of GaAs on the substrate occurs and is allowed to continue for a prescribed interval to yield the desired growth thickness. Thereafter, the melt is decanted from the surface of the substrate, by for example tipping the substrate so that the excess melt flows off, and then wiping any remaining tin from the surface of the epitaxial surface that is formed on the substrate. In this process, the gaseous atmosphere of hydrogen at 400 C. and atmospheres is changed to nitrogen, at about the same pressure and'temperature, when the melt is decanted from the substrate surface. Apparatus to provide the gaseous atmosphere at the proper conditions and to change the atmosphere from hydrogen to nitrogen, as described, is cumbersome and expensive and generally obscures observation of the process. Furthermore, impurities on the substrate surface are dissolved in the tin melt when the melt initially flows over the surface, and some of these impurities may enter the epitaxial film that is formed, introducing undesirable results.

The temperatures required for the above-described GaAs solution regrowth are sufficiently high that the hydrogen can react with surface oxides, thereby reducing the solution wetting problems. In the case of materials which require lower regrowth temperatures, the hydrogen becomes ineffective as an oxide reducing agent.

Accordingly, it is one object of the present invention t provide a method for growing an epitaxial layer of semiconductor material on a substrate that does not require apparatus for providing a controlled gaseous environment about the area of growth.

It is another object of the present invention to provide an improved method for forming an epitaxial layer of semiconductor material on a substrate.

It is another object of the present invention to provide an improved method for forming an epitaxial homojunction or heterojunction.

These and other objects of the present invention are accomplished by performing liquid phase growth or solution regrowth of an epitaxial layer of semiconductor material on a substrate of semiconductor material in a liquid 3,463,680 Patented Aug. 26, 1969 environment. More prticularly, the substrate and a melt containing the epitaxial material are immersed in a liquid, which preferably has a boiling temperture greater than the temperature of the melt. If the liquid boiling temperature is lower than the temperature of the melt, some problems are introduced, but they are not insurmountable. Furthermore, it is preferred that the selected liquid in which the solution regrowth occurs be substantially inert so as not to contaminate the substrate or the epitaxial layer which is grown. If possible, the liquid selected should be capable of removing undesirable impurities from the surface of the substrate, so that these impurities do not appear in either the substrate or the epitaxial layer that is formed.

In a particular embodiment of the present invention, an n-type epitaxial layer of InSb is formed on a p-type substrate of InSb while both are immersed in liquid stearic acid. The InSb substrate and a melt of In+InSb are preferably separated while both are immersed in the liquid stearic acid. When the melt and the liquid stearic acid are at about 300 C., the melt is caused to flow over the substrate and then the temperature is reduced. At about 200 to 250 C., the melt is poured off or decanted from the substrate and the newly formed epitaxial layer is wiped clean of the melt. The maximum temperature of the melt, the interval of time that the melt remains on the substrate and the temperature at which the melt is decanted from the substrate determine the thickness of the epitaxial layer formed.

In another embodiment of the invention, an n-type film of GaAs is formed on a p-type GaAs substrate while immersed in molten boric oxide, in much the same manner as described above.

These and other objects and features of the present invention are more clearly understood from the following specific description of embodiments. of the invention taken in conjunction with the figures in which:

FIG. 1 is a partially cutaway view of the furnace boat, which contains the substrate, the melt, and the covering liquid;

FIG. 2 illustrates the boat mounted to apparatus for heating the boat to provide the melt and for tipping the boat to pour the melt over the substrate and decant the melt from the substrate;

FIGS. 3 to 5 are section views of the boat at 3 different positions, illustrating how the boat is tipped, causing the melt to flow over the substrate and then tipped to cause the melt to decant from the substrate: and

FIG. 6 is a plot of temperature vs. time, which illusstrates the heating cycle and timing of the pouring and decanting steps.

Turning first to FIG. 1, there is shown a boron nitride boat 1 containing the substrate 2 and the melt 3 at the bottom 4 separated by ridge 5. The melt 3 may be, for example, a mixture of InSb and In selected from published solubility data and may be doped with a trace of, for exmple, Te, Se or S. The substrate 2, in this case, is p-type InSb.

After the substrate 2 and melt material 3 are placed in the bottom of the boron nitride boat 1, and a thermocouple 6 is attached to the bottom of the boat and connected to an indicator (not shown), the boat is partially filed with liquid stearic acid 7 which is deep enough to cover the melt when the boat is later tilted to pour and decant the melt over the substrate.

The purpose of the stearic acid is to keep the surface of the substrate and the melt clean and to insure wetting of the substrate surface by the melt and in general provide an inert environment to protect the substrate and melt before, during, and after the solution regrowth occurs.

The boat containing the substrate melt and stearic acid is mounted on the apparatus shown in FIG. 2. The

base 8 of the boat is held between clamps 9 and 11, firmly against the graphite heater strip 12, which runs from one clamp to the other. The clamps in turn connect to electrical conductors 13 and 14, which carry the electric current from a source (not shown) to the heater strip, which i turn heats the boat as necessary to melt the mixture of In and InSb which form the melt.

The electrical conductors 13 and 14 are contained in insulating shields 15 and 16, which mount to the tilting platform 17 carried on the pivoting arm 18, which is rotated so as to tilt the boat as necessary to pour and decant the melt by a drive mechanism which is not shown.

In operation, the boat is heated by the graphite heater strip to a temperature of about 300 C. This causes the In to liquefy and dissolve the InSb and the Te, Se or S dopant in contact with the In to form the desired melt. However, the InSb substrate, separated from the In, remains solid. This condition with the melt 3 fluid, the substrate solid and both immersed in the liquid stearic acid well below the boiling temperature 450 C. of the acid is illustrated in FIG. 3. Next, the boat is tilted, as shown in FIG. 4, so that the melt 3 flows over the substrate 2 and, thus, the melt coats the substrate.

Immediately thereafter, the graphite heater current is reduced so that the melt begins to cool. At the time when the melt first makes contact with the substrate, the melt is not quite saturated with InSb and a thin layer of the substrate surface goes into solution in the melt. When this occurs, any surface imperfections or damage in the substrate surface are removed. The dissolution of the surface of the substrate at these low temperatures occurs preferentially along a crystallographic plane, thereby leaving a perfectly planar interface between the substrate and the epitaxially grown layer. As the temperature of the melt is then reduced, the InSb solution is forced out of solution and grows in a uniform layer 18 on top of the substrate. The thickness of this uniform layer is controlled by controlling the temperature of the melt and the period of time the melt remains on the substrate, as well as the ratio of amounts of InSb and In in the melt. These factors can be obtained from published data by, for example, R. N. Hall in the Journal of the Electrochemical Soc., 110, 385 (1963).

When the temperature drops to about 200 to 250 C., as measured by the thermocouple 6 attached to the bottom of the boat, the boat is tilted back to the position illustrated in FIG. 5, causing the melt to decant from the surface of the substrate and, finally, the remaining melt is wiped off of the substrate, leaving the thin layer of n-type InSb 18 on the substrate. The stearic acid is then poured off before it solidifies.

The above regrowth is preferably done on a (100) crystal substrate face to enable cleaving perpendicular to the junction between the substrate and the regrowth layer.

FIG. 6 illustrates a typical heating cycle of temperature vs. time, showing the points at which the melt is poured over the substrate and the point at which it is decanted. As shown, the melt of In containing InSb and a trace of Te is poured over the substrate of InSb at a temperature of about 315 C. and is decanted at about 225 C. These temperatures are not critical and can be deviated from somewhat without producng any noticeable difference in the final product. The epitaxial layer produced is typically 100p. thick.

A similar technique is employed to form an epitaxial layer of GaAs or Ge on a substrate. For example, an epitaxial layer of n-type GaAs is formed on a p-type GaAs substrate. While both are immersed in molten boric oxide (above 360 C.). The method consists of placing a mixture of tin and GaAs at the bottom of a graphite boat, alongside a wafer of p-type GaAs, which is the substrate. Next, the graphite boat is heated to about 640 C., causing the GAas particles to dissolve in the tin, at one side, on the bottom of the boat. When the temperature reaches 640 C., which is well below the boiling temperature (1500 C.) of the boric oxide, the heat is turned off and the boat is tipped so that the molten tin pours over the GaAs substrate. At this point, the tin Which is nearly saturated with GaAs, dissolves GaAs from the substrate surface until solution equilibrium is establshed; and then upon further cooling, precipitation ofthe GaAs from the solution and epitaxial growth upon the substrate occurs. At about 400 C. or before the boric oxide solidifies, the boat is tipped to decant the molten tin, and any remaining tin on the surface of the epitaxial layer formed on the substrate is wiped off.

The InSb and GaAs epitaxial junctions described above are homojunctions, since the epitaxial layer and substrate are of the same semiconductor material; only the type may be different. Heterojunctions can be formed in the same way. For example, epitaxial InSb may be formed on a substrate of GaAs.

Other selections of semiconductor substrate material solution melts and suitable liquids in which substrate and melt can be immersed to carry out solution regrowth of a thin layer of semiconductor material on the substrate are listed on the table below. Some of these form homojunctions and some form heterojunctions.

Substrate material Homojunctions Gash in Ga Boric oxide. lnAs in In D0. InP in In Do. GaAs in Ga Do. Gal in Ga Do. Ge in Sn Do. Si in Sn.. Do. PbTe in Pb D0 IbSe in Pl). Do. IbS in Pb Do.

Ileterojunctions Gash InSb in In Stearic acid. GaAs InSb in In Stearic acid or boric oxide. Boric oxide. Stcaric acid.

The above table is but a partial list of combinations of substrate, melt and liquid medium materials for growing epitaxial layers to form homo or heterojunctions. Generally, the semiconductor materials in each case are Group IIIV, or Group IV-VI compounds or Group IV elements. The lead salts are particularly suitable.

The liquid mediums, boric oxide and stearic acid are suitable for the combinations indicated in the table, particularly when the regrowth is accomplished at ambient pressure (atmosphere). However, this does not preclude the use of other liquid mediums which can be ascertained by a simple survey of the art. For example, lower boiling temperature liquids can be used quite effectively if the system is placed under pressure to raise the liquid boiling temperature.

The melt in which the semiconductor material for the epitaxial layer is dissolved may be an alloy of two or more metals, instead of a single metal. The table above lists only a single metal for the melt in each case. The metal or alloy selected for the melt must, of course, be suitable medium for dissolving the semiconductor and any dopants added to it at a temperature above the melting temperature of the liquid medium and preferably below the boiling temperature of the medium. In addition, the substrate material is preferably slightly soluble in the melt and the melt materials do not degrade the junction formed.

One particular use of a semiconductor junction formed as described above is presented in the US. patent application Ser. No. 433,368, by Melngailis, Rediker and Phelan, entitled Laser Device. In that application, an InSb injection laser, called a plasma laser, fabricated as described herein, is mounted and energized so as to emit light in a direction normal to the plane of the junction. The plasma laser of a typical sort includes, for

example, a p-type base region of InSb which is hundreds of microns in each dimension. An N+ contact is formed on one face of this base and is highly polished. On the opposite face, a P+ layer is formed and this also is polished and then coated with a layer of reflecting metal. In operation, electrical contacts are made to N+, NP+ layers and lasing action takes place within the entire block of p-type base material in between. The optical cavity which supports this lasing action is defined by the polished surfaces of the N+ and P+ layers, and so it is seen that cur-rent injection is parallel to the laser axis and parallel to the emitted laser radiation. Such a plasma laser requires formation of a large, uniform, yet very thin n-type layer on one face of the base or substrate of p-type InSb. The method described herein has been found to be quite effective in forming this n-type layer on the substrate.

This concludes description of a number of embodiments of the present invention of methods for solution regrowth of a thin semiconductor layer on a substrate. The principal novelty of the invention is that the growth takes place while the solution and substrate are immersed in a liquid, which may be at ambient pressure. The .various embodiments described are intended to demonstrate applications of the invention and are not intended to limit the scope of the invention.

We claim:

1. A method for growing a layer of semiconductor material on a semiconductor substrate material, comprising the steps of:

coating said substrate material with a semiconductor or metallic melt containing said semiconductor material in solution therein, while said substrate material and said melt are immersed in a liquid which boils at a temperature greater than the temperature of said melt, and cooling to form an epitaxial layer of said semiconductor material on said substrate material.

2. A method as in claim 1, and in which:

said liquid is selected to provide an inert environment in which said growth occurs.

3. A method as in claim 1, and in which:

said liquid is selected to remove undesirable impurities from the surface of said substrate upon which said growth occurs.

4. A method as in claim 1, and in which:

said liquid is selected to provide an inert environment in which said growth occurs and to remove undesirable impurities from the surfaces of said substrate, upon which said growth occurs.

5. A method as in claim 1, and in which:

said substrate is semiconductor material of one conductivity type and said melt includes semiconductor material of another conductivity type in solution.

6. A method as in claim 1, and in which:

said melt includes in solution therein a material which forms a homojunction with said substrate material.

7. A method as in claim 1 and in which:

said melt includes in solution therein a material which forms a heterojunction with said substrate material.

8. A method as in claim 1 and in which:

said melt is composed of a metal, containing said semiconductor material in solution therein.

9. A method as in claim 1 and in which:

said melt is composed of an alloy of at least two metals containing said semiconductor material in solution therein.

10. A method as in claim 6 and in which:

said metal is In, said semiconductor material in solution is InSb, said substrate is InSb and said liquid is stearic acid.

11. A method as in claim 6 and in which:

said metal is Sn, said semiconductor material in solution is GaAs, said substrate is GaAs and said liquid is boric oxide.

12. A method as in claim 6 and in which:

said substrate material and said semiconductor material in solution are Group III-V compounds.

13. A method as in claim 6 and in which:

said substrate material and said semiconductor material in solution are Group IV-VI compounds.

14. A method as in claim 6 and in which:

said substrate material and said semiconductor material in solution are Group IV elements.

15. A method as in claim 7 and in which:

said substrate material and said semiconductor material in solution are Group III-V compounds.

16. A method as in claim 1 and in which, said melt includes a selected impurity in solution therein whereby said epitaxial layer contains said impurity in a selected concentration.

References Cited UNITED STATES PATENTS 3,272,342 10/1966 John et a1. 148-1.6 3,290,188 12/1966 Ross 148177 3,360,406 12./1967 Sumski 148--1.16 3,411,946 11/1968 Tramposch 1l7--201 OTHER REFERENCES Nelson, H., R.C.A. Review, December 1963, pp. 603- 615.

L. DEWAYNE RUTLEDGE, Primary Examiner P. WEINSTEIN, Assistant Examiner US. Cl. X.R. 

1. A METHOD FOR GROWING A LAYER OF SEMICONDUCTOR MATERIAL ON A SEMICONDUCTOR SUBSTRATE MATERIAL, COMPRISING THE STEPS OF: COATING SAID SUBSTRATE MATERIAL WITH A SEMICONDUCTOR OR METALLIC MELT CONTAINING SAID SEMICONDUCTOR MATERIAL IN SOLUTION THEREIN, WHILE SAID SUBSTRATE MATERIAL AND SAID MELT ARE IMMERSED IN A LIQUID WHICH BOILS AT A TEMPERATURE GREATER THAN THE TEMPERATURE OF SAID MELT, AND COOLING TO FORM AN EPITAXIAL LAYER OF SAID SEMICONDUCTOR MATERIAL ON SAID SUBSTRATE MATERIAL. 